The present invention relates to a method for decomposing a chlorine containing organic compound contained in an exhaust gas and a catalyst used for the method. More specifically, the present invention relates to a method for decomposing a chlorine containing organic compound contained in an exhaust gas in which method a chlorine containing organic compound, which is typified by dioxin contained in an exhaust gas from a refuse incinerator, can efficiently be decomposed even at a low temperature, and relates to a catalyst used for the method.
In recent years, a minute quantity of chlorine containing organic compounds possessing an extremely strong toxicity, for example, dioxins such as polychlorinated dibenzodioxins and polychlorinated dizenzofurans, and copulaner PCB (polychlorinated biphenyls) (hereinafter the compounds are sometimes referred to as DXNS), in addition to poisonous substances such as nitrogen oxides (NOx), sulfur oxides (SOx), and hydrogen chloride (HCl) are contained in exhaust gases from incinerators burning municipal refuses or industrial wastes. Thus, it is desired to establish technology for removing those compounds, as environmental contaminants. Further, it has lately come to be known that DXNs act as endocrine disruptors (the so-called environmental hormones) and that the DXNs are accumulated in breast milks up to a high concentration and adversely affect to newborn children. Thus, emission control for DXNs is being further strengthened. Accordingly, the importance of the technology for reducing the contents of DXNs in exhaust gases is more increased and various researches and developments are being carried out in many fields.
As the technology for decomposing DXNs contained in exhaust gases, thermal decomposition methods in the presence of a catalyst, oxidative decomposition methods with oxygen, and a method in which the activity for decomposing DXNs is promoted by adding ozone or hydrogen peroxide (Laid-open Japanese Patent Publication No. Hei 7-75720) are known. Among them, catalytic decomposition methods such as the thermal decomposition methods conducted in the presence of a catalyst and oxidative decomposition methods with oxygen are becoming mainstreams. Especially, commercialization of the oxydative decomposition methods with oxygen are widely being planned since the oxidative decomposition methods are not only high in the performance of decomposing DXNs but also have a capability of denitrating DXNs in addition.
Decomposition reaction of DXNs contained in exhaust gases by the thermal decomposition or oxidative decomposition with oxygen described above proceeds according to the following equation (1) or (2), respectively:
Thermal decomposition reaction Rxe2x80x94Cl (chlorine containing organic compound)xe2x86x92mH2+nC+pHCl+Rxe2x80x2xe2x80x94Clxe2x80x83xe2x80x83(1)
Oxidative decomposition reaction with oxygen Rxe2x80x94Cl (chlorine containing organic compound)+kO2xe2x86x92mCO2+nH2O+pHClxe2x80x83xe2x80x83(2)
wherein m, n, p, and k are integers, and R and Rxe2x80x2 represent skeletons of hydrocarbons.
As a result of the investigations conducted by the present inventors, it has first been found that there exists such a problem as the rate of the thermal decomposition reaction of the equation (1) described above is slow and thus a large quantity of a catalyst and a high temperature are required in order to obtain a performance necessary for commercially adopting the reaction. Also, it has been found that a rate of the oxidative decomposition reaction commercially adoptable can not be obtained whereas the rate of the oxidative decomposition reaction of the equation (2) with oxygen is higher than that of the thermal decomposition described above, and that a catalyst is deteriorated when SOx are contained in an exhaust gas.
An object of the present invention is to provide a method for decomposing a chlorine containing organic compound contained in an exhaust in which method a high decomposition ratio of dioxins can be obtained even at a low temperature, the amount of a catalyst to obtain a performance or capability necessary for commercially adopting the decomposition method can be reduced, and the effects by the SOx can be suppressed down to a minimum, and to provide a catalyst used for the method.
Another object of the present invention is to provide a method for treating a catalyst which was used for decomposing chlorine containing organic compounds (DXNs) (hereinafter a catalyst already used for decomposing DXNs contained in an exhaust gas to purify the gas is sometimes referred to as a used catalyst). More specifically, another object of the present invention is to provide a method in which the DXNs adhered to a used catalyst can efficiently be decomposed and removed, and after which method was completed, safe working conditions at the time of conducting routine checkups of or taking out a used catalyst can be secured and a used catalyst can safely and readily be dumped or recycled.
In order to achieve the objects described above, various studies were conducted by the present inventors on thermal decomposition methods of DXNs in the presence of a conventional catalyst and on oxidative decomposition methods of DXNs with oxygen to find that the thermal or oxidative decomposition methods have the following problems:
That is, first, a high reaction rate of decomposing dioxins can not be obtained unless a temperature at which the catalytic reactions are initiated is high and the reactions are conducted at a high temperature. Especially, the thermal decomposition reaction is slow in reaction rate, and a reaction rate commercially adoptable can not be obtained unless the reaction temperature is as high as 300xc2x0 C. or higher. Whereas a decomposition ratio of DXNs can be increased if the reaction temperature was raised, a re-synthesis reaction of dioxins from hydrocarbons, carbon monoxide (CO), chlorine compounds and others contained in an exhaust gas, and a polychlorination (isomerization) reaction having a higher toxicity proceed, leading to the generation of dioxins as opposed to the intention of reducing an amount of dioxins.
Second, the use of a large quantity of a catalyst is necessary since both the thermal decomposition reaction and oxidative decomposition reaction with oxygen are slow in reaction rate. It becomes a heavy burden, to small and medium sized municipalities operating refuse incinerators, to use a large quantity of an expensive catalyst. Moreover, when the amount of a catalyst is increased, a risk of generating dioxins tends to increase. That is when a catalyst exists in an exhaust gas, the increase in the amount of the catalyst apparently causes an increase of decomposition ratio of dioxins, since the thermal decomposition or oxidative decomposition reaction with oxygen, and such a re-synthesis reaction of DXNs as described above occur at the same time, the rate of the decomposition reaction is higher than that of the re-synthesis reaction, and thus the differential rate between both reactions becomes the amount of dioxins to be reduced. On the other hand, however, a risk that dioxins are re-synthesized increases, and there exists a risk that large quantities of dioxins are generated when the catalyst was deteriorated.
Third, a problem that decomposition ratio of dioxins is likely to be affected by sulfur oxides (SOx) contained in an exhaust gas can be mentioned. That is, generation of SOx at the time of burning refuses or industrial wastes is inevitable. Especially, a catalyst tends lately to be used at a lower temperature in order to avoid the re-synthesis of dioxins described above, deterioration of the catalyst by SOx becomes more remarkable at such a low temperature region, and thus it is not easy to obtain a high dioxins decomposition ratio according to conventional technology in which sufficient countermeasures against SOx are not taken.
Accordingly, diligent investigations were further carried out by the present inventors on conditions for efficiently decompose DXNs contained in exhaust gases to make them innoxious, and on a catalyst to be used at that time, based on some knowledge obtained by the studies in the early stage described above. As a result of the diligent investigations, first, the nitrogen oxides, particularly NO2 usually contained in exhaust gases together with DXNs received attention, and then it was found that it is adequate for achieving an object of the present invention to oxidatively decompose DXNs by using the NO2 contained in an exhaust gas, or by using the NOx added anew into an exhaust gas. Also, with respect to another object of the present invention, it was found that DXNs adhered to a catalyst can be decomposed by contacting a used catalyst with a gas containing NOx, and further that decomposition of DXNs can be accelerated by making a specific catalyst comprising a titanium oxide, molybdenum oxide, and vanadium oxide be present at that time. These discoveries led to the achievements of the present invention.
The present invention is summarized as follows:
(1) A method for decomposing a chlorine containing organic compound contained in an exhaust gas comprising reacting the chlorine containing organic compound contained in the exhaust gas with nitrogen dioxide which is contained in the exhaust gas or added anew from the outside into the exhaust gas, at 100 to 450xc2x0 in the presence of a catalyst such that the concentration of nitrogen dioxide in the exhaust gas after the termination of the reaction is higher than 1 ppm, to oxidatively decompose the chlorine containing organic compound with the nitrogen dioxide.
(2) The method for decomposing a chlorine containing organic compound contained in an exhaust gas recited in paragraph (1) above wherein the catalyst comprises a titanium oxide, molybdenum oxide, and vanadium oxide, the contents of titanium (Ti), molybdenum (Mo), and vanadium (V) in the catalyst being in the range of 99 to 70/0.5 to 15/0.5 to 15 in terms of atomic ratio, respectively.
(3) The method for decomposing a chlorine containing organic compound contained in an exhaust gas recited in paragraph (1) or (2) above wherein the exhaust gas contains a sulfur oxide.
(4) A method for decomposing a chlorine containing organic compound contained in an exhaust gas comprising introducing ammonia into the exhaust gas containing the chlorine containing organic compound and then contacting the exhaust gas with a catalyst comprising a titanium oxide, molybdenum oxide, and vanadium oxide the contents of titanium (Ti), molybdenum (Mo), and vanadium (V) in which catalyst being in the range of 99 to 70/0.5 to 15/0.5 to 15 in terms of atomic ratio, respectively, at 100 to 450xc2x0 C. to oxidatively decompose the chlorine containing organic compound with nitrogen dioxide which is contained in the exhaust gas or added anew from the outside into the exhaust gas and concentration of which nitrogen dioxide is higher than that of the chlorine containing organic compound, and to decompose the nitrogen oxide with the ammonia such that the nitrogen oxide remains at a concentration of higher than 1 ppm.
(5) The method for decomposing a chlorine containing organic compound contained in an exhaust gas recited in paragraph (4) above wherein a part of the reaction for decomposing the nitrogen oxide with the ammonia is carried out in advance before the exhaust gas is contacted with the catalyst recited in paragraph (2) above.
(6) The method for decomposing a chlorine containing organic compound contained in an exhaust gas recited in paragraph (4) or (5) above wherein the exhaust gas contains a sulfur oxide.
(7) A catalyst used for the method recited in paragraph (1) or (4) above and comprising a titanium oxide, molybdenum oxide, and vanadium oxide, the contents of titanium (Ti), molybdenum (Mo), and vanadium (V) in the catalyst being in the range of 99 to 70/0.5 to 15/0.5 to 15 in terms of atomic ratio, respectively.
(8) A method for treating a catalyst used for decomposing a chlorine containing organic compound comprising contacting a catalyst which was used for purifying an exhaust gas containing a chlorine containing organic compound, with a gas containing nitrogen dioxide to oxidatively decompose the chlorine containing organic compound adhered to the catalyst, with the nitrogen dioxide.
(9) The method for treating a catalyst used for decomposing a chlorine containing organic compound recited in paragraph
(8) above wherein the contact of the catalyst with the gas containing nitrogen dioxide is carried out at a temperature lower than 250xc2x0 C.
(10) The method for treating a catalyst used for decomposing a chlorine containing organic compound recited in paragraph (8) or (9) above wherein the catalyst is contacted with the gas containing nitrogen dioxide after the dust adhered to the catalyst was removed by washing the catalyst in advance.
(11) The method for treating a catalyst used for decomposing a chlorine containing organic compound recited in paragraph
(10) above wherein a waste water produced at the time of removing the dust in advance is heated to obtain steam (vapor), a gas containing nitrogen dioxide is added into the steam obtained from the waste water, and then the mixture of the gas with the steam is contacted with the catalyst.
(12) The method for treating a catalyst used for decomposing a chlorine containing organic compound recited in any one of paragraphs (8) to (11) above wherein the decomposition of the chlorine containing organic compound adhered to the catalyst, with nitrogen dioxide is carried out in an apparatus having the catalyst placed or disposed therein.
(13) The method for treating a catalyst used for decomposing a chlorine containing organic compound recited in any one of paragraphs (8) to (12) above wherein the catalyst comprises, as a main component, a titanium oxide and further comprises vanadium, and molybdenum or tungsten.
The method of the present invention is to oxidatively decompose chlorine containing organic compounds (DXNs) contained in an exhaust gas with nitrogen dioxide (NO2) by reacting the nitrogen dioxide already contained in the exhaust gas or added from the outside anew to the exhaust gas with the chlorine containing organic compound in the presence of a catalyst having a specific chemical composition at a prescribed temperature such that the concentration of nitrogen dioxide in the exhaust gas after the reaction was terminated is higher than 1 ppm, preferably higher than 3 ppm.
The catalyst used in the present invention and having a specific chemical composition comprises, as essential components, titanium oxide (TiO2), molybdenum oxide (MoO3), and vanadium oxide (V2O5), and the mixing ratio of titanium (Ti), molybdenum (Mo), and vanadium (V) is in the range of 99 to 70/0.5 to 15/0.5 to 15 in terms of atomic ratio
A catalyst comprising only titanium and vanadium exhibits a certain extent of the activity for decomposing DXNs with nitrogen dioxide. However, the activity of the catalyst for decomposing DXNs with nitrogen dioxide is considerably increased when molybdenum coexists in the catalyst. Further, when titanium, molybdenum, and vanadium coexist in a catalyst, lowering of the activity of the catalyst for decomposing DXNs with nitrogen dioxide under coexistence of SOx can be avoided and thus a high decomposition activity of the catalyst is obtained even at low temperature regions at which a remarkable deterioration of the catalyst with SOx is apt to occur. For instance, whereas the activity of a titanium-vanadium (Tixe2x80x94V) or titanium-vanadium-tungsten (Tixe2x80x94Vxe2x80x94W) type catalyst for decomposing a halogen containing organic compound with oxygen at 200xc2x0 C. is almost completely lost by the presence of 50 ppm of SOx, the activity of the three components catalyst of titanium-molybdenum-vanadium (Tixe2x80x94Moxe2x80x94V) of the present invention for decomposing the halogen containing organic compound with nitrogen dioxide is little reduced even if the same concentration of SOx existed.
Catalyst of the present invention is calcined in somewhere in the process for preparing the catalyst, at 300 to 650xc2x0 C., preferably 400 to 600xc2x0 C. When the calcination temperature is too low, organic compounds contained in the raw materials for the catalyst are not decomposed, formation of a complex through mutual mixing of the oxides becomes insufficient, and thus high catalyst performances can hardly be obtained. On the other hand, when the calcination temperature is too high, molybdenum oxide in a catalyst composition is sublimed, titanium oxide is sintered, and thus catalyst performances are deteriorated.
Catalyst of the present invention can be obtained through such a known method as a kneading method, impregnation method, and wash coating method by using, as raw materials, a titanium oxide obtained by one of various kind of such processes as a sulfuric acid process and chlorine process or a water-containing titanium oxide such as ortho- or meta-titanic acid, and oxides, ammonium salts, or mineral acid salts such as sulfuric acid salts of molybdenum or vanadium. Also, it is possible to add such an inorganic or organic bonding agent as inorganic fibers and colloidal silica in the process for preparing a catalyst to increase the strength of a molded catalyst. As to the shape of catalyst, a catalyst carried on a filter cloth of a bag filter, or on a particle-like or honeycomb-like carrier made of a ceramic in addition to a catalyst molded into particle-like, plate-like, or honeycomb-like can be mentioned.
In the present invention, a reaction in which DXNs in an exhaust gas are oxidatively decomposed with NO2 is expressed as follows:
Oxidative decomposition reaction of a chlorine containing organic compound with NO2 Rxe2x80x94Cl (chlorine containing organic compound)+kNO2xe2x86x92mCO2+nH2O+pHCl+kNOxe2x80x83xe2x80x83(3)
wherein m, n, p, and k are integers, and R represents a hydrocarbon skeleton.
Reaction temperature is 100 to 450xc2x0 C., preferably 120 to 250xc2x0 C., and most desirably 120 to 200xc2x0C. The oxidative decomposition reaction of DXNs with NO2 begins at about 120xc2x0 C., and proceeds at a far low temperature compared with a thermal decomposition reaction and oxidation reaction with oxygen both of which belong to conventional technology. Reaction rate at a low temperature region lower than 250xc2x0 C. is several times to several ten times as high as that in the conventional methods, and DXNs are efficiently decomposed even such a low temperature region. Although the re-synthesis temperature of dioxins is being said to be 250 to 350xc2x0 C., dioxins can efficiently be decomposed according to the decomposition method using a catalyst of the present invention having an excellent activity at a low temperature while avoiding the re-synthesis temperature region of dioxin.
In the present invention, NO2 may be that which was added anew from the outside in addition to that already contained in an exhaust gas. Since the equilibrium between NO and NO2 in the presence of oxygen at the reaction temperature region described above inclines to NO2 side the NO contained in an exhaust gas is successively converted into NO2, and thus the NO2 may be that which was formed by oxidation of the NO contained in an exhaust gas. It is sufficient that the concentration of the NO2 is higher than that of DXNs.
Catalyst of the present invention also has an activity as a catalyst for reducing NOx contained in an exhaust gas with NH3 in addition to the activity for decomposing DXNs. Accordingly, it is possible to perform the decomposition of DXNs and the decomposition of NOx at the same time or with the one being closely behind the other in the presence of one catalyst.